chore(audiobooks): migration 141 no-op for media_folders.type

Sub-plan 1 originally reserved migration 141 to add a 'kind' column to
media_libraries discriminating audiobook/podcast libraries. Discovery
audit (sub-plan 1 Task 1) found that the actual table is media_folders
and it already has a type text NOT NULL column with no CHECK constraint
or enum, so 'audiobooks' and 'podcasts' can be added as future values
without DDL.

Landing this migration as a documented no-op preserves the version
numbering audit trail and pins the decision in git history. The
matching down migration is also a no-op.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/141_media_folders_kind_noop.down.sql

@@ -0,0 +1,3 @@
+-- See 141_media_folders_kind_noop.up.sql — intentionally empty.
+
+SELECT 1;

migrations/141_media_folders_kind_noop.up.sql

@@ -0,0 +1,19 @@
+-- Intentionally a no-op migration.
+--
+-- The audiobooks absorption design spec assumed silo needed to add a
+-- 'kind' column to a hypothetical media_libraries table to discriminate
+-- between movies/tv/audiobooks/podcasts at the library level. Discovery
+-- found that the actual table is media_folders and it already has a
+-- type text NOT NULL column carrying that role (existing values:
+-- 'movies', 'series', 'mixed').
+--
+-- There is no CHECK constraint or enum on media_folders.type, so adding
+-- 'audiobooks' or 'podcasts' as future values requires no DDL — the
+-- scanner branches in sub-plan 2 will simply read those values from
+-- media_folders.type the same way they do for 'movies' and 'series'.
+--
+-- This migration is preserved as a numbered no-op so the audit trail
+-- shows the decision rather than leaving a numbering gap. The matching
+-- .down.sql is also a no-op.
+
+SELECT 1;